Recently, printing systems utilizing lasers to reproduce information on laser sensitive mediums have been announced by various manufacturers. Typically, the system includes a laser for generating a laser beam, a modulator, such as an acousto-optic modulator, for modulating the laser beam in accordance with input information to be reproduced, a multi-faceted scanner for scanning the modulated laser beam across a medium on a line to line basis, various optical components to focus and align the laser beam onto the medium, the laser sensitive recording medium itself and, if necessary, a means for developing the information in human readable form. A typical representation of printing systems utilizing lasers is shown, for example, in U.S. Pat. No. 3,922,485, the patent disclosing the use of a xerographic drum as the recording medium and means for developing the latent electrostatic image formed on the drum.
In some printing systems it is required that a cylindrical lens be interposed between the modulated laser beam and the scanner to provide a fan fold light output beam which fully illuminates at least one facet of the scanner. The cylindrical lens is preferably aligned with the laser beam to redistribute the energy of the modulated laser beam in a manner whereby the energy distribution of the laser beam incident on the recording medium is substantially summetrical about the start of scan and end of scan positions of the medium, maximum energy ideally occurring at the center of the scanline. This is particularly important in those systems which utilize a xerographic recording medium.
In general, the laser portion of the overall printing system is regarded as the "weak link" in that the laser would be the first component to fail or in some way become inoperative. The accepted approach, from a field service standpoint, to make the printing system operative, would be to remove the defective laser and replace it with another laser, either a new laser or a repaired laser. In regards to the configuration discussed above, it has been determined that the relative positioning of the laser beam with respect to the cylindrical lens is extremely critical, this critically affecting the energy distribution of the laser beam which scans the medium. If the laser beam is moved with respect to the cylindrical lens a small distance, say for example 0.001 of a inch in the scanning direction, the beam energy incident on the polygon facet will shift a substantial fraction of the facet aperture in the scanning or tangential direction, the displacement being determined by the magnification factor of the cylindrical lens which in turn will effect the beam energy distribution at the surface of the recording medium. The relatively tight tolerance required between the laser beam and the cylindrical lens has made it extremely time consuming and costly to replace the laser independently of the cylindrical lens and maintain the necessary tight tolerances therebetween.
A laser printing system wherein tolerances between the laser beam and certain of the down stream optical elements is maintained within acceptable limits without the attendant increased costs and time delays which would otherwise occur is disclosed in copending application Ser. No. 911,021, now U.S. Pat. No. 4,230,902 assigned to the assignee of the present invention. In particular, the invention disclosed therein provides a technique for maintaining the relative alignment between a laser beam, in a modular laser printing system, and one of the optical elements associated therewith and in particuar, a cylindrical lens, in a manner such that the energy distribution of the beam as it is scanned across a recording medium is maintained at an optimum value. The alignment between the cylindrical lens and the laser beam is maintained by mounting the laser and cylindrical lens to a common support member within a module, each element being fixed with respect to the common support member. If the laser fails or otherwise becomes defective, or if any of the other elements in the module similarly fails, the whole module is capable of being removed and replaced with a similar module. In other words, a field replaceable module is provided such that the required alignment tolerances between the cylindrical lens and the laser beam is maintained. The failed laser in the removed module can be repaired or replaced with a new one, the laser then being mounted in the module, the initially removed module thereafter being capable of being reutilized in the laser printing system. The module output beam, when the module or similar module is replaced in the printing system, retains its relative alignment with system components external to the module.
As set forth hereinabove, a misalignment of the laser beam at the scanner (polygon) will cause a large variation in the beam intensity at the photoreceptor as the scanner rotates to scan the beam thereacross. This is particularly serious when the scanner aperture is smaller or comparable in size to the laser beam diameter.
Although the technique described in the aforementioned copending application substantially eliminates the alignment problems caused in the prior art due to the field replacement of the individual lasers, extremely tight tolerances in the operative laser scanning system has caused difficulties in maintaining system specifications in the field.
What is desired, therefore, is a relatively simple and precise adjustment procedure which will allow the optical system to be realigned by field (or manufacturing line) personnel thus increasing the effective life of the laser as well as the performance of the laser scanning system.